Shrinkable stress-relief cone and method



May 2, 1 7 c. J. OATESS ETAL 3,317,655

SHRINKABLE STRESS-RELIEF CONE AND METHOD Filed Feb. 15, 1965 I 7 I i 223 g&\\\\\\ 24 INVENTORS CECIL J. oATEss Y WALTER J. PLATE MEI/1% United States Patent Oiiice 3,317,655 Patented May 2, 1967 3.3171655 SHRINKABLE STRESS-RELIEF CONE AND METHOD Cecil J. Oatess, Marion, Ind., and Walter J. Plate, Rye,

N.Y., assignors. to Anaconda Wire and Cable Company,

a corporation of Delaware Filed Feb. 15, 1965. Ser. No. 432,584 9 Claims. (Cl. 174-73) ABSTRACT OF THE DISCLOSURE In a high-voltage electrical cable, the stress-reliefcone, which is required at terminations, is preformed in a heat-shrinkable condition, inserted over the cable, and shrunk down to form a tight, void-free fit.

Our invention relates to stress-relief cones and particularly to stress-relief cones for use on shielded electrical cables.

In the use of high-voltage electrical apparatus, particularly shielded electrical cable, it is known, where the shield is terminated, thus creating a discontinuity in the electrical field surrounding the cable or other apparatus, to apply a stress-relief cone adjacent to the discontinuity. The cone 'has the effect of increasing the insulation thickness at the critical area and thus reducing the stress concentration, and the shield is extended over the cone so that it has a much larger radius of curvature at the point of termination. Stress-relief cones have conventionally been formed by the hand wrapping of a large number of layers of dielectric tape. This is not only time consuming but it requires considerable skill, since the cone should be built to a precise contour and, particularly, since it must be free from voids, spaces, and

abrupt surface irregularities. It has "been suggested to use molded or cast stress-relief cones of rubber or plastic such as polyethylene or epoxy resin but these have not been found practical for several reasons. In the first place there is so much random variation in the insulation diameters of commercial cables that premolded cones can not be expected to fit closely over the insulation of the particularsection of a cable being terminated. Also, since no voids, whatever, are tolerable between the outer surface of insulation and the inner bore of the stress cone, the fit is necessarily so close that it would be impossible to work the length of insulated conductor into the bore of the stress cone. Again, it should be realized that the outer surface of the insulation of a cable, particularly one from which a jacket and shield has been removed at the installation site, is bound to be irregular so that it will not fit a premolded stress cone with the necessary precision and freedom from voids. If the stress cones are molded or cast over the conductor insulation it is necessary to have special equipment present in the field, and, of course, to wait while the molded or cast member is setting. This is always inconvenient and in many cases impossible, because of the awkward conditions surrounding an installation site.

All the objections and difiiculties above enumerated have been overcome by our invention comprising preformed stress-relief members that can be shrunk down Over an insulated conductor after they have been placed in the proper position. Following widespread practice the expression stress-relief cone is used throughout this application to designate the members herein described although they are not always geometrically conical in shape. They are, however, always distinguished by the fact that they have walls that increase in thickness so as to provide an increased wall of insulation at a conductor discontinuity or an increased cross-section for distributing the electrical line of force.

By our invention we have provided a means of reducing the electrical stress concentration at a discontinuity in an insulated conductor that requires no specially developed skill for its application.

Our invention comprises a stress cone that will fit snugly and free from voids around an insulated conductor in spite of manufacturing variations in dimension and irregularities in the surface.

The stress cone of our invention has the further advantage that the same size of cone can be used for a range of conductor diameters.

We have invented a stress-relief cone comprising a tubular body having a portion defining a bore for the insertion of an insulated electrical conductor. The stressrelief cone is shrinkable by the application of heat whereby it can be shrunk down snugly around the conductor. The body is continuous in circumference, and it increases substantially in thickness lengthwise from one end. Our stress-relief cone may be highly dielectric with a resistivity in excess of megohms per cm. or, in other embodiments it may be semiconducting with a resistivity between 1 and 10 ohms per cm.

Our stress-relief cone may have an electrically conducting surface-covering between one end and the section of maximum thickness.

We have invented the method of reducing the electrical stress concentration at a discontinuity in an insulated electrical conductor having an electric shield comprising the steps of removing the shield from the conductor to a point along its length and inserting an unshielded length of the conductor adjacent to this point into an oversized, shrinkable stress-relief cone. The cone has a continuous tubular body increasing substantially in thickness from an end adjacent to the point where the shield is removed, to a section of maximum thickness and, in one embodiment of our invention, has an electrically conducting outer surface between the abovementioned end and the section of maximum thickness which, in our method, is connected electrically to the shield. The stress-relief cone is then shrunk down snugly around the conductor. In another embodiment of our method an electrically conducting layer is applied over the stress-relief cone after it 'has been shrunk down. In this method, also, the conducting layer is electrically connected to the terminated shield. Our method also envisages an embodiment wherein are included steps of inserting the stress-relief cone with its conducting surface layer-into an oversized shrinkable protective cover and shrinking the cover down snugly over the conducting layer.

Alternatively, for certain applications at lesser voltage we apply an oversized semiconducting stress-relief cone over the conductor and the cut end of the shield and then shrink it down to a snug fit.

A more thorough understanding of our invention may be gained from a study of the appended drawing.

In the drawing:

FIGURE 1 is a lengthwise section of a stress-relief cone of our invention.

FIGURE 2 is a lengthwise section of a cover for the cone of FIGURE 1.

FIGURE 3 is a section of a cable termination made by the method of our invention.

FIGURE 4 is a section of a stress-relief cone made in accordance with another embodiment of our invention.

The stress-relief cone shown in section in FIGURE 1 and indicated generally by the numeral 10 is comprised of a tapered tubular body 11 of a heat shrinkable composition of which several known types will hereinafter be described. The body 11 has a portion defining a cylindrical bore 12 into which an insulated conductor may be inserted; and the stress-relief cone 10, as shown in its un-shrunk condition in FIGURE 1, is oversized for the intended conductor so that the latter can be inserted easily. The stress-relief cone has ends 1 3, 14 to which it is tapered, the end 13 being intended for fitting adjacent to the cut edge of a cable shield. From the end 13 the wall 11 increases in thickness to a section 16 at which it has its maximum thickness which is always substantially greater than the thickness of the wall at the end 13. This is an essential feature of the stress-relief cone since it is the purpose of the cone to present a section of greater diameter for the purpose of attenuating the electrical stresses. The surfaces of the cone 10 are surfaces of revolution and a surface 17 between the end 13 and the section 16 is conical. 'For use at very high voltage it is known that the contour of the surface 17 should be logarithmic and it is one of the advantages of premolded stress-relief cones that the contour can be more precisely determined than it can be for a taped construction made by hand in the field.

A number of known shrinkable substances can be used for the fabrication of the stress-relief cone 10. The bore of the stress-relief cone is formed to a diameter slightly smaller than the least insulated conductor diameter for which it is intended. The stresssrelief cone is then heated, radially stretched in the heated condition by means of an expanding mandrel to an oversized condition, and, while still in the oversized condition, chilled. Thereafter the stress-relief cone will retain its stretched dimensions until it is reheated, preferably to the temperature at which it was originally expanded. It is also known to make shrinkable plastics, such as polyethylene and polyethylene-blends, by irradiation. Suitable heatshrinkable substances include polyvinyl chloride; neoprene (particularly neoprene that has been blended with about 17% of its weight of polyethylene); the silicone-rubber, polyethylene blend comprising -25 parts of polyethylene per 100 parts of silicone-rubber disclosed in application Ser. No. 299,781 assigned to the assignee of the instant invention; and butyl-rubber (particularly butylrubber blended with about polyethylene).

In one embodiment of our invention the conical outer surface of the stressarelief cone 10 is rendered electrically conducting, such as by the application of a conducting lacquer layer 15. This is best accomplished while the stress-relief cone is in the oversized condition. Suitable conducting coatings are well known and do not constitute a novel feature of the instant invention. The stress-relief cones so far described are comprised of dielectric substances and constitute an extension of the cable insulation. Such substances have a resistivity in excess of 100 megohms per cm. and usually measured in millions of megohms. When a dielectric substance is used for the stress-relief cone, a conducting surface is applied over the stress-relief cone. However, it has also been known to make the stress-relief cones themselves semiconducting, that is, to use a substance with a resistivity of l to 10 ohms per cm. This is done in the embodiment :shown in FIGURE 4 where no conducting surface is applied over a stress-relief cone 1 8 comprised of semiconducting rubber. Methods of compounding normally dielectric materials to be semiconducting are well known and comprise the inclusion of conducting particles such as acetylene black in the compositions. About of acetylene black included in a shrinkable neoprene compound, for example, will render it semiconducting while still retaining the heat shrinking characteristics. The stress-relief cone 18 has a spherical contour and is appliied over an insulated cable 19 in such a manner that it covers the cut end 21 of a metallic cable shield 22. A bore 23 of the stress-relief cone 18 is shown in its preshrunk, oversized condition and the stress-relief cone 18 -.will be shrunk QWX snugly over the insulated conductor 4 19 and the shield 22 by the application of a hot air blast 24 from a heater 26.

The method of our invention may best be understood by reference to FIGURE 3 wherein is shown a stressrelief cone 27 after it has been applied to a cable termination. A cable, indicated generally by the numeral 28, has an insulated conductor 29 comprised of a metal strand 31 and a wall of cable insulation 32. A metallic shield 33 is cut at an end 34 leaving an unshielded portion 36 of the insulated conductor which is inserted into a bore 37 prior to shrinking and at a time when the stress-relief cone is still oversized. The stress-relief cone 27 is shrunk down snugly around the insulated conductor 29 with an end 38 adjacent to the cut end 34 of the shield. A wrapping of copper mesh or other conducting tape 39 is then wrapped over the stress-relief cone 27 between the end 38 and a section 41 where the wall thickness of the stress-relief cone 27 is a maximum. To assure electrical contact between the shield 33 and the shield formed by the conducting tape 39 a turn of the latter is made around the former and soldered thereto. Thereafter an oversized heat-shrinkable protective cover 42 (FIGURE 2), preferably comprised of neoprene, is fitted so as to surround the stress-relief cone 27 and shield 3-9. The cover 42 has a main body portion 43 shaped to fit snugly over the stress-relief cone 27, and tubular extensions 44, 46 which fit respectively over a length of the shield 33 and the insulation 32. The cover 42 can also be applied over a stress-relief cone that has a preapplied conducting surface layer such as the layer 15.

The embodiments of our invention hereinabove described are understood to be exemplary rather than definitive. Other embodiments and modifications coming within the full scope of our invention are defined by the following claims.

We claim:

1. A stress-relief cone comprising:

(A) a tubular body having a portion defining a bore for the insertion of an insulated electrical conductor,

(B) said stress-relief cone being shrinkable by the application of heat whereby it can be shrunk down snugly around said conductor,

(C) said body being (a) continuous in circumference and (b) increasing substantially in thickness lengthwise from an end thereof.

2. The stress-relief cone of claim 1 having a resistivity in excess of megohms per cm.

3. The stress-relief cone of claim 1 having a resistivity between 1 and 10 ohms per cm.

4. A stress-relief cone comprising:

(A) a tubular body having a portion defining a bore for the insertion of an insulated electrical conductor,

(B) said stress-relief cone being shrinkable by the application of heat whereby it can be shrunk down snugly around said conductor,

(C) said body being (a) continuous in circumference and (b) increasing substantially in thickness lengthwise gradually from an end thereof to a section of maximum thickness, and

(D) an electrically conducting surface-covering surrounding a substantial portion of said body from said end to said section.

5. The method of reducing the electrical stress concentration at a discontinuity in an insulated electrical conductor having an electric shield comprising the steps of:

(A) removing the shield to a point along the length of said conductor,

(B) inserting an unshielded length of said conductor adjacent to said point into an oversized shrinkable stress-relief cone having a continuous tubular body increasing substantially in thickness from an end adjacent to said point to a section of maximum thickness.

(C) shrinking said stress-relief cone down snugly around said conductor,

(D) applying an electrically conducting layer over said stress-relief cone between said shield and said section.

6. The method of reducing the electrical stress concentration at a discontinuity in an insulated electrical conductor having an electric shield, comprising the steps of:

(A) removing the shield from said conductor to a point along the length of said conductor,

(B) inserting an unshielded length of said conductor adjacent to said point into an oversized shrinkable stress-relief cone having a continuous tubular body increasing substantially in thickness from an end adjacent to said point to a body section of maximum thickness and having an electrically conducting outer surface between said end and said section,

(C) shrinking said stress-relief cone down snugly around said conductor, and

(D) electrically connecting said shield to said conducting outer surface.

7. The method of reducing the electrical stress concentration at a discontinuity in an insulated electrical conductor having an electric shield comprising the steps of:

(A) removing the shield from said conductor to a point along the length thereof,

(B) inserting an unshielded length of said conductor and said section into an oversized, shrinkable, semiconducting stress-relief cone having a body increasing substantially in thickness from the ends thereof, and

(C) shrinking said stress-relief cone down snugly around said conductor.

8. The method of reducing the electrical stress concentration at a discontinuity in an insulated electrical conductor having an electric shield comprising the steps of:

(A) removing the shield to a point along the length of said conductor,

(B) inserting an unshielded length of said conductor adjacent to said point into an oversized shrinkable stress-relief cone having a continuous tubular body increasing substantially in thickness from an end adjacent to said point to a body section of maximum thickness,

(C) shrinking said stress-relief cone down snugly around said conductor,

(D) applying an electrically conducting layer over said stress-relief cone between said end and said section, said layer being electrically connected to said shield.

9. The method of reducing the electrical stress concentration at a discontinuity in an insulated electrical conductor having an electric shield comprising the steps of:

(A) removing the shield to a point along the length of said conductor,

(B) inserting an unshielded length of said conductor adjacent to said point into an oversized shrinkable stress-relief cone having a continuous tubular body increasing substantially in thickness from an end adjacent to said point to a body section of maximum thickness,

(C) shrinking said stress-relief cone down snugly around said conductor,

(D) applying an electrically conducting layer over said stress-relief cone between said end and said section, said layer being electrically connected to said shield,

(E) surrounding said stress-relief cone and said layer with an oversized shrinkable protective cover, and

(F) shrinking said cover down snugly over said layer.

References Cited by the Examiner UNITED STATES PATENTS 1,987,971 1/1935 Peterson 174-73 2,036,414 4/1936 Jore 174-73 2,789,154 4/1957 Peterson 174-73 3,035,113 5/1962 Danchuk 17477 X 3,210,460 10/ 1965 Suelmann 1747'3 FOREIGN PATENTS 1,168,900 9/1958 France.

297,571 6/ 1954 Switzerland.

LARAMIE E. ASKIN, Primary Examiner. 

1. A STRESS-RELIEF CONE COMPRISING: (A) A TUBULAR BODY HAVING A PORTION DEFINING A BORE FOR THE INSERTION OF AN INSULATED ELECTRICAL CONDUCTOR, (B) SAID STRESS-RELIEF CONE BEING SHRINKABLE BY THE APPLICATION OF HEAT WHEREBY IT CAN BE SHRUNK DOWN SNUGLY AROUND SAID CONDUCTOR, (C) SAID BODY BEING (A) CONTINUOUS IN CIRCUMFERENCE AND (B) INCREASING SUBSTANTIALLY IN THICKNESS LENGTHWISE FROM THE AN END THEREOF.
 7. THE METHOD OF REDUCING THE ELECTRICAL STRESS CONCENTRATION AT A DISCONTINUITY IN AN INSULATED ELECTRICAL CONDUCTOR HAVING AN ELECTRICAL SHIELD COMPRISING THE STEPS OF: (A) REMOVING THE SHIELD FROM SAID CONDUCTOR TO A POINT ALONG THE LENGTH THEREOF, (B) INSERTING AN UNSHIELDED LENGTH OF SAID CONDUCTOR AND SAID SECTION INTO AN OVERSIZED, SHRINKABLE, SEMICONDUCTING STRESS-RELIEF CONE HAVING A BODY INCREASING SUBSTANTIALLY IN THICKNESS FROM THE ENDS THEREOF, AND (C) SHRINKING SAID STRESS-RELIEF CONE DOWN SNUGLY AROUND SAID CONDUCTOR. 